1. Field of the Invention
The present invention relates to a turbine ring for a gas turbine.
2. Summary of the Prior Art
Nos. FR-A-2 540 937, FR-A-2 540 938, FR-A-2 540 939 and FR-A-2 371 575, all described turbine rings for gas turbines, each such ring including an annular carrier secured to the inside of the turbine casing, and a ring, which is at least partially formed of a ceramic, abradable material, and which is secured to the inside of the said annular carrier. In most of these constructions, the annular carrier is of a metallic material and, as a result of the substantial difference existing between the respective coefficients of expansion of the metallic materials and of the ceramic materials, the ring of ceramic material must be built up from segments which are independent of one another, and interconnected by their respective ends in such a way as to enable the radius of the ring to follow variations in the radius of the annular carrier, as a function of the differential temperatures which the latter assumes for various operational ratings of the turbine, thus avoiding the ring of ceramic material being subjected to stresses which are incompatible with the mechanical strength of the material on which it is made.
No. FR-A-2 559 834 describes, in the preamble, numerous disadvantages associated with the use of a ring of ceramic material, built up from juxtapose multiple segments. Furthermore, it is indicated that the disadvantages may be at least partially overcome by constructing the annular carrier also of a ceramic material, and manufacturing the abradable ring in a single piece. In one preferred embodiment of the turbine ring described in this French patent application, the dimensioning is, furthermore, such that the annular carrier exerts, when cold, on the abradable ring, a predetermined precompression force in such a way as to cancel out or even invert the forces at the normal operational temperature of the turbine. In this previously proposed technique there is thus avoided the necessity of providing for metallic interconnections between two parts built up on the one hand from a metallic material and on the other of a ceramic material. In practice the interconnections between the abradable ring and its annular carrier are provided, in accordance with this prior proposal, by radial screw threaded members, screwed into inserts locked into the abradable rings. The relative complexity of the structure is compensated by the facility which it for disassembly of the ring, for example for the purpose of replacing its abradable part.
In several of the prior patent applications, which have been referred to hereinbefore, means are provided for controlling the temperature of the component parts of the turbine ring, such means comprising for example means to provide a flow of cooling air derived from the compressor of the turbine plant. Such cooling means are generally arranged so that they act indiscriminantly on the two main components of the turbine ring, namely the annular carrier and the element or elements of abradable ceramic material. As a result, the temperature gradient between the inner faces and the outer faces of the abradable ring, for example, is very substantial and, in itself, gives rise to stresses which can reduce its useful life.